In recent years, high-output linear power amplifiers compensated for distortion using a feedforward have been used for base station apparatus of mobile communication equipment.
First, the configuration of a conventional feedforward amplifier will be explained with reference to FIG. 12, which is a block diagram of the conventional feedforward amplifier.
In FIG. 12, reference numeral 1 denotes an input terminal; 2, an output terminal; 3 and 9, power dividers; 4 and 10, power combiners; 5 and 12, vector adjustors; 7, a main amplifier; 8 and 11, delay circuits; 13, an auxiliary amplifier. Furthermore, reference characters a to k and m marked on power dividers 3 and 9, and power combiners 4 and 10 denote their respective ports.
An input signal including a plurality of carrier frequency components is applied from input terminal 1 and is divided into two portions by the power divider 3 at port b and port c, respectively.
The signal output from the port b is passed through the vector adjustor 5 and amplified by the main amplifier 7. The resultant signal is passed through the power divider 9 and delay circuit 11 and input to port j of the power combiner 4. At this time, due to non-linearity of the main amplifier 7, a signal including not only the carrier frequency components but also distortion components originated by intermodulation are input. Furthermore part of the output signal of the main amplifier 7 is extracted from port f of the power divider 9 and input to port h of the power combiner 10.
On the other hand, the signal output from the port c is passed through the delay circuit 8 and input to port g of the power combiner 10.
Here, by adjusting the vector adjustor 5 and delay circuit 8 so that the carrier frequency components of the signals input to the port g and port h have the same amplitude and opposite phases, a signal containing only the distortion components with the carrier frequency components canceled out is output from the port i.
Then, the signal output from the port i is passed through the vector adjustor 12, amplified by the auxiliary amplifier 13 and input to the port k of the power combiner 4.
Here, by adjusting the vector adjustor 12 and delay circuit 11 so that the distortion components of the signals input to the port j and port k have the same amplitude and opposite phases, a signal including only the carrier frequency components with the distortion components canceled out is output from the port m of the power combiner 4 to the output terminal 2.
Different from the above-described distortion compensation system, there is known a pre-distortion system (pre-distortion compensation system). The configuration of the pre-distortion circuit will be explained with reference to FIG. 13, which is a block diagram of the pre-distortion circuit.
In FIG. 13, reference numeral 61 denotes a power divider; 62, a power combine; 63, a delay circuit; 69, a vector adjustor; 72, a distortion generation circuit. The power divider 61, power combiner 62, delay circuit 63, vector adjustor 69 and distortion generation circuit 72 constitute a pre-distortion circuit 6. Furthermore, reference character n marked on the power combiner 62 denotes a port.
In operation, pre-distortion system suppresses distortion by inputting signals having the same frequency components as those of the distortion components generated at the main amplifier 7 into the main amplifier 7 with an appropriate amplitude/phase relationship.
The input signal including the plurality of carrier frequency components input from the input terminal (the terminal on the left side in FIG. 13) is divided into two portions by the power divider 61.
One portion of the two divided output signals is input to the power combiner 62 through the delay circuit 63. The other portion of output signal is input to the distortion generation circuit 72, which outputs a signal with the same frequency component as the distortion component generated at the main amplifier 7. The signal output at the distortion generation circuit 72 is input to the power combiner 62 through the vector adjustor 69 and combined with the output signal from the delay circuit 63. At this time, the vector adjustor 69 is adjusted in such a way as to have an appropriate amplitude/phase relationship with the input signal.
The signal combined at the power combiner 62 is output from the port n and input to the main amplifier 7.
However, with the feedforward amplifier in FIG. 12, as the carrier band widens, a difference is produced in the conditions under which the vector adjustor 12 suppresses the distortion component generated on the higher frequency side and the distortion component generated on the lower frequency side than the carrier band to a maximum degree. That is, if the vector adjustor 12 is adjusted in such a way as to suppress the distortion component on the high frequency side to a maximum, the distortion component on the low frequency side is not suppressed sufficiently, and vice versa. Thus, the vector adjustor 12 has been adjusted so far so that the distortion component on the high frequency side and the distortion component on the low frequency side reach the same level.
Moreover, with the pre-distortion circuit in FIG. 13, as the carrier band widens, it is also difficult to suppress the distortion component on the higher frequency side and the distortion component on the lower frequency side than the carrier band simultaneously. Thus, suppression has been performed so far in such a way that the distortion component on the high frequency side and the distortion component on the low frequency side reach the same level, but the result of this shows that it is only possible to suppress the distortion components on the order of a few dB.
A feedforward amplifier using both the pre-distortion system together with feedforward system has previously performed suppression in such a way that the distortion component on the high frequency side and the distortion component on the low frequency side reach the same level. This is the case especially when the distortion component generated at the main amplifier 7 has an unbalance between the high frequency side and low frequency side of the carrier band. Thus, distortion has not been suppressed efficiently.